CNN — An award-winning author has whipped up controversy by describing Catherine, the Duchess of Cambridge, as a "machine-made" doll with a "plastic smile" who lacks the personality and human frailty shown by Princess Diana.

Hilary Mantel, who has twice won the Man Booker Prize for her historical novels set in Tudor times, made the comments in a talk for the London Review of Books titled "Royal Bodies," looking at how royal lives play out under the public gaze.

The furor over her comments came as Catherine carried out her first official engagement since she announced her pregnancy, with a visit to an addiction treatment clinic in London.

While there, she told reporters it would be "unnatural" if she was not nervous about having a child.

The duchess, who is patron of the charity that runs the clinic, Action on Addiction, was briefly hospitalized late last year with acute morning sickness. She and husband Prince William are expecting a baby in July.

In her lecture, Mantel described how Catherine's public image was first defined by her clothes, and then her pregnancy.

Before she became a mother-to-be, "I saw Kate becoming a jointed doll on which certain rags are hung," Mantel said, and "a shop-window mannequin, with no personality of her own, entirely defined by what she wore." Now, she will be portrayed as "her only point and purpose being to give birth."

The author also suggested the chief attribute brought by Catherine to her royal marriage was good manners, adding that the duchess "appeared to have been designed by a committee and built by craftsmen, with a perfect plastic smile and the spindles of her limbs hand-turned and gloss-varnished."

And Mantel contrasted that with the potential for disaster that Diana, the late Princess of Wales and mother to princes William and Harry, carried with her, saying Kate was "irreproachable: as painfully thin as anyone could wish, without quirks, without oddities, without the risk of the emergence of character."

It is very early days for Kate as a royal, and her image is being carefully managed, Barnett said. "She has ample time to develop her public persona and become a fully-fledged role model if needs be," she said.

Nor does Catherine have the luxury of answering criticism, she added. "As a fully paid-up member of the royal family, she can only respond by doing the very same thing Mantel has criticized her for: staying quiet."

The Daily Mail also blasted Mantel's comments as "an astonishing and venomous attack."

Mantel's literary agent, Bill Hamilton, said Tuesday the author had no comment on the controversy over her lecture. "The article speaks for itself," he said.

Mantel last year became the first woman to twice win the Man Booker Prize, for her novel "Bring Up the Bodies." She previously won in 2009 with "Wolf Hall."

Her novels focus on the Tudors -- and in the lecture, she draws a parallel between the current fascination with Catherine's body and the public scrutiny under which the wives of Henry VIII sought to produce a male heir.

Last week, an Italian magazine provoked an uproar in the UK media when it published pictures of a bikini-clad Catherine on vacation, with her "baby bump" visible. Palace officials said they were disappointed by what they said was a clear breach of privacy.

"Long before Kate's big news was announced, the tabloids wanted to look inside her to see if she was pregnant," Mantel said.

"Cheerful curiosity can easily become cruelty. It can easily become fatal. We don't cut off the heads of royal ladies these days, but we do sacrifice them, and we did memorably drive one to destruction a scant generation ago."

The institution of monarchy may be irrational, Mantel added, but those outside don't have to forget their principles as they observe the "entertainment" the royals provide.

"I'm not asking for censorship. I'm not asking for pious humbug and smarmy reverence. I'm asking us to back off and not be brutes," she said.

Emily Bell, director of the Tow Center for Digital Journalism at the Columbia Journalism School, defended Mantel via Twitter, saying the furor exemplified the author's point about the media's role.

"The Middleton row is caused by the Mail turning Hilary Mantel's rather wonderful lecture into an example of exactly what Mantel warns of..." she said.

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